> > Just some food for thought how this all fits together rather> > neatly.> > It's actually the membership system that glues it all together. The> dlm is just another service.

Membership is one of the lowest level and high privileged inputs to thewhole picture, of course.

However, "membership" is already a pretty broad term, and one mustclearly state what one is talking about. So we're clearly focused onnode membership here, which is a special case of group membership; thetop-level, sort of.

Then every node has it's local view of node membership, constructedtypically from observing node heartbeats.

Then the nodes communicate to reach concensus on the coordinatedmembership, which will usually be a set of nodes with full N:Nconnectivity (via the cluster messaging mechanism); and they'll alsousually aim to identify the largest possible set.

Eventually, there'll be a membership view which also implies certainshared data integrity guarantees if appropriate (ie, fencing in case anode didn't go down cleanly, and granting access on a clean join).

These steps but the last one usually happen completely internal to themembership layer; the last one requires coordination already, becausethe fencing layer itself might need recovery before it can fencesomething after a node failure.

And then there's quorum computation.

Certainly you could also try looking at it from a membership-centricangle, but the piece which coordinates the recovery of the variouscomponents which makes sure the right kind of membership events aredelivered in the proper order, and errors during component recovery areappropriately handled, is, I think, pretty much distinct from the"membership" and a higher level component.

So I'm not sure I'd buy "the membership is what glues it all together"on eBay even for a low starting bid.

Sincerely, Lars Marowsky-Brée <lmb@suse.de>

-- High Availability & ClusteringSUSE Labs, Research and DevelopmentSUSE LINUX Products GmbH - A Novell Business